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Welding procedure for welding HT4140 to mild steel

Anyone developed a welding procedure for welding HT4140 to mild steel? I need to fabricate a pair of spindles for a road race car and the pin we are using is 2" dia. HT4140, and the spindle uprights are mild steel.

This is not a welding tip, but a press fit one. Most pins are machined to have a press fit. (about .003 in.) You put it in the freezer for about an hour, and heat the other part. I know this isn't what you asked for, but thought it may help.

4140

I've welded a 4140 CrMo shaft to an unknown mild steel shaft that was guessed to be a mechanical grade, 70 ksi or so, it was a section about 8" in diameter and solid. The part was a broken pivot shaft that holds the track frame onto the main frame of a large mining drill.

The procedure I used was provided to me as a guideline. After initial jig setup, I pre heated both sections of material to within 375F - 425F, tack welded the components, then completed the weld with a 80xx type lo-hyd electrode, not letting the inter-pass temps go over 500F. Interpass heating was used to keep the temp up. After completion, the weldment was wrapped with fire blankets to allow slow controlled cooling.
I used a 2% nickel alloy 80xx low-hyd rod for the root and subsequent passes until the build up was sufficient to us LA-T91-C60 NI1 .045 flux cored wire to fill out the rest. This was completed in the horizontally fixed position so all welding was done from 6 oclock to 12. The weld repair was sucsessful and has had to work at temps below -40C, suffering mostly impact and torsion type stresses.
I've also sucsessfully welded 4140 to mild in much smaller on-the-bench arrangments, and generally followed the same type of procedure. 350-450F preheat, controlled interpass temps, and let cool slowly.

The use of a needle scaler to remove slag and inter-pass pein the weldment to remove stress was also employed in some of the repairs and so far there have been no come-backs.

I have always used wire from Rockmount Research when welding 4140 to mild steel. You can find them at www.weldit.com. All good advice you have been given so far. If this was brought to me to weld I would look into tig welding it first & then go from there.